Is there a way to disable composite effects on already rendered frames?
Unless you saved the images before post processing, there is no way: once an image has been processed, flattened and saved into an output format, you can't de-construct it and revert it to a previous stage.


  You will need to render again.


But before you do that please read on.

As a piece of advise: you can use a file output node to save each frame as is being rendered before post processing.

Also never render an animation as video. Render an image sequence and encode later. 

If you care for quality use OpenEXR as the render format. That way you can keep your information, and passes undistorted and linear.

There is a reason why Post-production exists. If you have saved all of your frames before any effects, then you can brig those rendered frames into the compositor and experiment different looks and effects in a non-destructive manner, without having to re-render the original rendered layers every single time. 

Rad: Is there a way to export each rendered layer as its own image?
 and How can I render all render passes to files in Cycles?